1. Field of the Invention
The present invention relates generally to a grain milling machine for milling grains, such as rice, wheat or barley. More specifically, the invention relates to a vertical grain milling machine for milling grains while the grains are forcibly fed into a lower portion of a milling section to be discharged from an upper portion of the milling section.
2. Related Background Art
Grain milling machines are divided broadly into friction type grain milling machines and grinding type grain milling machines. The friction type grain milling machines are designed to cause grains to pass through a grain milling cylinder at a predetermined pressure to peel rice bran layers from the surfaces of the grains by frictional force or scraping force applied between the grains. Such friction type grain milling machines are widely used for making brown rice into usual milled rice.
The grinding type grain milling machines are designed to mill grains by grinding the surfaces of the grains by means of abrasive grains provided on a grinding roll, which is arranged in a cylindrical punched steel plate having slits to rotate at a high speed, while the grains pass through a gap of about 10 mm between the steel plate and the grinding roll. Such grinding type grain milling machines are used for processing rice for sake brewery, which is obtained by removing rice bran layers and a part of starch layers of brown rice, and for polishing grains having hard rice bran layers, such as heat or barley.
Conventionally, the above described grinding type grain milling machine uses an emery roll 70 shown in FIG. 7 as a grinding roll. On the surface of the emery roll 70, abrasive grains 71 called "emery" (carborundum) are formed. The emery roll 70 is obtained by adding clay, feldspar powders, a binder and water to silicon carbide to form a mixture, sufficiently drying the mixture, and then, heating and sintering the dried mixture at a temperature of about 1400.degree. C. The shape of the emery roll 70 is cylindrical, a screw-shaped, a truncated-cone-shaped or the like, and designed to change the peripheral velocity by changing the diameter thereof.
In the above described conventional grinding type grain milling machine having the emery roll 70, there is a problem in that the depths of the surface flaws of an object to be ground are not constant due to the irregularities of the abrasive grains 71, so that the water absorbing characteristic of milled rice is uneven during rice cooking, thereby making cooked rice grain uneven, and damaging chewing taste.
In addition, since the grinding force deteriorates due to friction force of the emery (abrasive grains 71) and so forth, there are problems in that it is required to frequently exchange the emery roll 70 to make the maintenance of the grain milling machiscreenroublesome and to increase the running costs.
Moreover, some of emery rolls 70 can not obtain sufficient grinding force. Therefore, in case of grain milling which requires to grind a part of starch layers, such as grain milling of rice of old crop and rice for sake brewery, there are problems in that it is required to repeat steps about five to seven times to finish required grain milling, so that the efficiency of grain milling operation is required.